1. Field of the Invention
The present invention relates to the field of neurology. More particularly, the invention is drawn toward the regulation of G protein functions, and the related nerve cell growth cone collapse.
2. Description of the Background Art
It has been suspected since the turn of the century, and proven more recently, that the reason injured neurons do not regenerate in the CNS is primarily because of inhibitory influences from the microenvironment, rather than from an intrinsic inability to grow (S. Ramon y Cajal, New Ideas on the Structure of the Nervous System in Man and Vertebrate (MIT Press, Cambridge, Mass., 1990); S. David and A. J. Agayo, Science 214:931 (1981); R. J. Keynes and G. M. W. Cook, Cur. Opin. Neurobiol. 2:55 (1992)). Included among the inhibitory influences are components of myelin and activities associated with the surfaces of brain membranes (P. Caroni and M. E. Schwab, J. Cell Biol. 106:1281 (1988); J. A. Raper and J. P. Kapfhammer, Neuron 2:21 (1990)), although their molecular identities are unknown. These factors can be assayed in vitro because they cause growth cone collapse, a feature which correlates with inhibition of nerve growth (J. A. Raper and J. P. Kapfhammer, Neuron 2:21 (1990); J. A. Davies et al., Neuron 2:11 (1990); E. C. Cox et al., Neuron 2:31 (1990)). The collapse of growth involves G proteins (M. Igarashi et al., Science 259:77 (1993)). Pertussis toxin (PTX) blocks the inhibitory influences of the aforementioned factors (M. Igarashi et al., Science 259:77 (1993)), but its irreversibility and toxicity significantly limits its usefulness in inhibiting growth cone collapse.
GAP-43 is a protein associated with the inner surface of growth cone membranes, and is believed to function in the regulation of nerve growth and/or nerve terminal plasticity (J. I. Benowitz and A. Routtenberg, Trends Neurosci. 10:527 (1987); J. H. P. Skene, Annu. Rev. Neurosci. 12:127 (1989); S. M. Strittmatter and M. C. Fishman, BioEssays 13:127 (1991)). Recently, it has been shown that GAP-43 acts as a G protein stimulator, by enhancement of guanine nucleotide exchange (S. M. Strittmatter et al., Nature 344:836 (1990); S. M. Strittmatter et al., J. Biol. Chem. 266:22465 (1991)). When injected into oocytes, GAP-43 can enhance the responsiveness to ligands for G protein-coupled receptors many-fold, suggesting that it interacts at the level of G proteins and the coupling of G proteins to receptors (S. M. Strittmatter et al., Proc. Natl. Acad. Sci. USA 90:5327 (1993)).
Peptides corresponding to the GAP-43 amino terminus also enhance G protein activity, suggesting that this is the active domain of GAP-43 in this interaction (S. M. Strittmatter et al., Nature 344:836 (1990)). Although it is not known how such peptides might enter cells, even longer peptides have been reported to enter nerve cells and affect nerve growth (E. Bloch-Gallego et al., J. Cell Biol. 120:485 (1993)).